Sky view factors estimation using a 3d-gis extension

The sky view factor indicates the relationship between the visible area of the sky and the area covered by urban structures. A method of sky view factors (SVF) estimation for urban analysis was developed and implemented in a Geographical Information System (GIS). In order to validate this tool, this paper presents a comparison of actual SVF with the results obtained by simulating the SVF in particular areas of a medium sized city with the developed tool. The simulation with this tool pointed out many advantages, such as: reliability; flexibility and ability to predict sky view factors in future scenarios. INTRODUCTION Urban climate and its heat islands phenomena are comprehensively studied nowadays. Yet, many topics on this subject remain important issues to be examined. The urban climate has a high variability and it is generally warmer and less windy than rural climate. It depends on many characteristics such as topography, regional wind speed, urban morphology and many other factors. Considering the urban canopy layer, i.e, the air contained within urban street canyon, its radiation budget depends on thermal characteristics of materials and geometry of the surroundings. Buildings trap energy reducing the urban long wave heat loss and generating the urban heat island. Among the main aspects causing the urban heat island are the physical characteristics and the spatial relationships of urban features. They have a direct influence on the thermal performance of buildings. The less a surface has visibility to the sky, the slower its cooling ability. Therefore, a thermal parameter called sky view factor (SVF) is one of the main heat island causes. Heat islands present their highest intensity in specific times of the day and they are usually greater in nocturnal than in daytime hours. Urban heat islands have significant influence on cities’ energy consumption, however, there are only relatively few works dealing with their relationship. As shown by Santamouris et al. (2001) and Williamson and Erell (2001), there is not only a reduction on heating energy consumption, but also an increase on cooling load of buildings related to the spatial distribution of the urban heat island. Hence, this thermal phenomenon cannot be neglected on thermal simulation. Taking into account those relevant points of the urban thermal environment, a research project named CEU (which in Portuguese stands for “Consumo de Energia Urbano”, or Urban Energy Consumption) is being undertaken at the State University of Sao Paulo in Bauru, Brazil. The CEU Project studies the relevance of urban geometry on the electric energy consumption of medium sized cities, seeking the identification of patterns of urban energy consumption for urban planning proposals. Moreover, the project works in collaboration with another research effort conducted by Silva (2003), which deals with the issue of urban sustainability on medium-sized Brazilian and Portuguese cities (The Urban Sustainability Project). Considering the usual technological limitations of countries in the same stage of development as that of Brazil, CEU Project steps include the development, improvement and application of planning supporting tools, (STModule), as shown in Figure 1. One of the main contributions to the Urban Sustainability Project in this case is the identification of common urban environmental indicators for small and medium-sized cities of both countries. In the present stage of the research, a tool that allows an easy and quick way of calculating and representing sky view factors has been created under the CEU Project framework, as described by Souza et al. (2003). That tool, called 3DSkyView Extension, is mainly an algorithm written in Avenue to work with ArcView GIS 3.2 and its 3D Analyst extension switched on. That tool transforms coordinates of polygons into stereographic and orthographic coordinates to allow sky view factors estimation. As such, it represents an innovation created by the authors, what is here evaluated. As a result of the ST-Module, the 3DSkyView Extension was developed by researchers of the Sao Paulo State University in Bauru, Brazil, and of the University of Minho in Braga, Portugal. After several tests with hypothetical examples, the tool described by Souza et al. (2003) is here applied in a real city scenario. Its potential as a sky view factor simulator is now evaluated under those conditions.